Project Summary Rotator cuff tears are a significant clinical problem, resulting in 30% of all visits to orthopaedic surgeons and over 150,000 surgical procedures per year. Rotator cuff tears occur frequently in the middle-to-late aged population and can cause severe pain, disability and activity limitations Exercise therapy is typically prescribed for initial treatment of rotator cuff tears, and is often effective at relieving symptoms. However, it fails in 25-50% of cases, requiring surgical intervention. The factors contributing to a successful outcome of exercise therapy are not precisely known, buy may include the size and location of the rotator cuff tear, range of motion, strength, pain level, and repetitive loading due to abnormal glenohumeral kinematics. Therefore, the overall goal of the proposed project is to conduct a prospective observational cohort study to describe the effects of exercise therapy in terms of patient-reported and structural outcomes, as well as to identify predictors of these outcomes in patients with an isolated supraspinatus tear. We will quantify in vivo glenohumeral joint motion; tear size/location using ultrasound, muscle strength; and patient-reported outcomes in subjects before and after an exercise therapy program. Our preliminary data have shown that: 1) the location of full-thickness rotator cuff tears in the supraspinatus tendon has an effect on tear propagation and 2) exercise therapy can improve in vivo glenohumeral joint stability during arm elevation. The project will address three specific aims: Aim 1) Identify factors that predict patient-reported outcomes and tear propagation after a standardized 12- week supervised exercise therapy program. The ability of glenohumeral joint contact path length; initial tear size; internal/external range of motion and muscle degeneration to predict patient-reported outcomes and tear size/propagation at 0, 6 and 12 months after exercise therapy will be examined; Aim 2) Investigate effects of tear location on tear propagation and glenohumeral kinematics; Aim 3) Determine the relationship between tear propagation and failure of the exercise program to improve patient-reported outcomes and/or undergo surgery within two years. The resulting data will be used to design future randomized clinical trials to determine whether patients should initially undergo non-operative or surgical treatment, based on patient- reported and clinician-measured parameters as well as biomechanical measurements at time of diagnosis. The long-term goal of this research is the development of individualized treatment plans for improving outcomes while reducing time and expense of care. Restoration of shoulder function after rotator cuff tear is essential for maintaining a healthy, active lifestyle, particularly in our aging population.